OFDMA
Orthogonal frequency-division multiplexing (OFDM) is a modulation technique used at the physical layer (PHY) of a number of wireless networks, e.g., networks designed according to the well known IEEE 802.11a/g and IEEE 802.16/16e standards. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple access scheme based on OFDM. In OFDMA, separate sets of orthogonal tones (subchannels) and time slots are allocated to multiple transceivers (users or mobile stations) so that the transceivers can communicate concurrently. OFDMA is widely adopted in many next generation cellular systems such as 3GPP Long Term Evolution (LTE) and IEEE 802.16m due to its effectiveness and flexibility in radio resource allocation.
OFDMA Resource Allocation
The radio spectrum is a scarce resource in wireless communications, and therefore an efficient use of it is needed. The rapid growth of wireless applications and subscriber users have called for a good radio resource management (RRM) scheme that can increase the network capacity and, from a commercial point of view, save deployment cost. Consequently, developing an effective radio resource allocation scheme for OFDMA is of significant interest for industry.
The fundamental challenge in resource allocation is the inequality between the scarce spectrum that is available, and the vast area to be covered and large number of users to be served. In other words, the same frequency spectrum must be reused in multiple geographical areas or cells. This will inevitably incur inter-cell interference (ICI), when users or mobile stations (MSs) in adjacent cells use the same spectrum. In fact, ICI has been shown to be the predominant performance-limiting factor for wireless cellular networks. As a result, a significant amount of research has been devoted to developing ICI-aware radio resource allocation for cellular networks
In order to maximize the spectral efficiency, frequency reuse factor of one is used in OFDMA cell deployment, i.e., the same spectrum is reused in each and every cell. Unfortunately, this high spectrum efficiency is also accompanied by high detrimental ICI. Therefore, a good ICI management scheme on top of OFDMA is needed to leverage the OFDMA technology.
OFDMA Resource allocation has been studied extensively for tile single-cell case. Most of existing methods focus on the optimization of power or throughput under the assumption that each MS would use different subchannel(s) in order to avoid intra-cell interference. Another key assumption in single-cell resource allocation is that the base station (BS) has the full knowledge of channel signal-to-noise ratio (SNR) of link between itself and every MS. In the downlink (i.e., transmission from BS to MS), this SNR is normally estimated by the MS and fed back to the BS. In the uplink (i.e., transmission from MS to BS), BS can estimate the SNR directly based upon the signal it receives from every MS. Its counterpart in the multi-cell scenario, namely the signal-to-interference-and-noise ratio (SINR), is however more difficult to obtain because the interference can come from multiple cells and would depend on a variety of factors such as distance, location, and occupied channel status of interferers which are unknown before resource allocation. This results in mutual dependency of ICI and complicates the resource allocation problem. Thus, a practical multi-cell resource allocation scheme that does not require global and perfect knowledge of SINR is highly desirable.
Inter-Cell Interference Coordination (ICIC)
ICIC is a technique that can effectively reduce ICI in cell-edge regions. It is achieved by allocating disjoint channel resources to cell-edge MSs that belong to different cells. Because cell-edge MSs are most prone to high ICI, the overall ICI can be substantially reduced by judicious coordination of channel allocation among cell-edge MSs. More specifically, ICIC reduces the number of interferers and/or the “damage” each interferer causes. The latter can be achieved by, for instance, allocating the same resource to geographically farther apart MSs so that due to path loss the interference is mitigated.
However, ICIC solely based on avoiding resource collision for cell-edge users can offer only limited performance gain in the downlink communications, because it overlooks the interference caused by transmission from the BS to cell-center MSs. The embodiments of the invention aim to propose a holistic channel allocation scheme where all MSs, cell-center and cell-edge alike, are taken into ICI management consideration.
Spatial Division Multiple Access (SDMA)
SDMA provides multi-user channel access by using multiple-input multiple-output (MIMO) techniques with precoding and multi-user scheduling. SDMA exploits spatial information of the location of MSs within the cell. With SDMA, the radiation patterns of the signals are adapted to obtain a highest gain in a particular direction. This is often called beam forming or beam steering. BSs that support SDMA transmit signals to multiple users concurrently using the same resources. Thus, SDMA can increase network capacity.
Base Station Cooperation (BSC)
Base station cooperation (BSC) allows multiple BSs to transmit signals to multiple MSs concurrently sharing the same resource (i.e., time and frequency). It utilizes the SDMA technique for BSs to send signals to MSs cooperatively and is specifically used in cell-edge MSs that are within the transmission ranges of multiple BSs. Thanks to cooperation, the interfering signal becomes part of the useful signal. Thus, BSC has two advantages: provision of spatial diversity and ICI reduction.
Diversity Set
Typically, each MS is registered at and communicates with one BS, which is called the anchor (or serving) BS. However, in some scenarios such as handover, The MS can concurrently communicate with more than one BS. A diversity set is defined in the IEEE 802.16e standard to serve this purpose. The diversity set track of the anchor BS and neighboring BSs that are within the communication range of the MS. The information in the diversity set is maintained and updated at the MS as well as the BS, and will be used in the graph-based method in this invention.
Graph-Based Framework in Prior Channel Allocation
The channel allocation problem in conventional (non-OFDMA) cellular and mesh networks has been solved using a graph coloring approach. In the conventional problem formulation, each node in the graph corresponds to a BS or an access point (AP) in the network to which channels are allocated. The edge connecting two nodes represents the potential co-channel interference, which typically corresponds to the geographical proximity of the BSs. Then, the channel allocation problem that respects the interference constraints becomes the node coloring problem, where two interfering nodes should not have the same color, i.e., use the same channel.
In conventional networks, if two adjacent base stations transmit at the same time using the same spectrum, then they cause interference to each other in the MSs. Thus, in the conventional graph, all that is required is to ensure that adjacent nodes representing BSs have different colors.
That solution is in applicable to OFDMA networks, where the frequency reuse factor is one, and all BS do use the same spectrum. In addition, conventional graphs do not consider technologies, such ICIC and BSC, as described above.